The use of carbon discs as a friction element in aircraft wheel and brake assemblies have gained acceptance as a way of increasing the payload of an aircraft.
In U.S. Pat. No. 3,639,197, it is disclosed how a continuous carbon fiber can provide a carbon disc with the structural unity needed to absorb repeated braking torque. Unfortunately, carbon oxidizes in an oxidizing atmosphere such as air and when the temperature of the carbon is above 800.degree. F. the oxidization increases very rapidly. When carbon friction discs are used in aircraft braking systems, thermal conditions above 800.degree. F. are often experienced. The swept or mated areas of such carbon discs shield each other from the oxidizing effects caused by air, however, the non-swept or mated areas are fully exposed to the oxidizing effects resulting from being exposed to air. Prolonged oxidization of the exposed areas of the carbon discs results in a loss of thermal heat sink capacity and structural strength which ultimately can cause a brake failure.
In U.S. Pat. No. 3,914,508 a method is disclosed for protecting a carbon substrate in a moist environment from oxidation by coating a selected surface thereof with a boron and metallic mixture. However, it is difficult to maintain a uniform coating thickness over the entire peripheral surface and as a result where the coating is limited or absent, deterioration of the carbon substrate often takes place after repeated brake applications.
In a further attempt to protect a carbon friction material, a metal driving ring as disclosed in U.S. Pat. No. 3,473,637, was pressed on to the outer periphery of a carbon disc to establish a unitary structure and thereby prevent oxidation of the non-swept or rubbed area of the carbon disc. During frictional operation when the thermal energy produced is low, such protection is effective. However, as the thermal energy generated during a brake application increases, the unity of the carbon material and steel ring change in direct proportion to the differences in their thermal coefficients of expansion placing a stress on the carbon disc. As a result of such stress, after a repeated number of frictional engagements at high temperatures, structural defects can occur along the periphery of the carbon disc. Thereafter, oxygen in the surrounding air can enter and degrade the underlying carbon friction disc.
U.S. Pat. No. 3,972,395 discloses a protection member which matches the coefficient of friction of the carbon friction disc. The protection member, which includes a woven carbon sheath and a protecting screen, is bonded to the peripheral non-swept surface to protect the underlying carbon friction disc. In order to protect the driving slots on the rotor member, a reinforcing plate is attached to the peripheral surface by a driving pin that extends through the carbon friction disc. Unfortunately, these driving pins are located in a high stress area and under some extreme conditions a structural failure may occur in the area of the driving pins.
In copending U.S. Patent Application Ser. No. 958,213, a series of flexible metal cap members surround the driving splines of a carbon disc. The metal cap members have projections which extend into the root section between the driving splines. These projections are connected together to establish a continuous ring of protection for the peripheral surface of the carbon disc. During a brake application, adjacent carbon disc contact the metal cap members to establish a barrier that prevents the passage of air to carbon discs that could degrade the driving splines through oxidization.